Closed-captioning (CC) refers to visual captioning on a video display that superimposes subtitles onto the display. “Captions” aim to describe all significant audio content, as well as “non-speech information,” such as the identity of speakers and their manner of speaking Additionally, captions may indicate sounds other than dialog that are associated with an aspect of the video. In many broadcast applications, closed-captioning information is embedded within the broadcast signal for decoding and for display along with selected video content of the broadcast signal. The term “closed” in closed-captioning refers to the selective display of captions for only those who decode or activate them. This is distinguished from open captions, where the captions are visible to all viewers.
For live programs, spoken words included in the program's soundtrack may be transcribed by an operator, and the phonetic output can be translated into text and displayed on the screen. In some instances, the transcript may be available in advance of the broadcast, and the captions may be displayed from the available transcript information. For prerecorded programs and home videos, the audio may be transcribed and the captions can be prepared, positioned, and timed in advance.
For National Television Systems Committee (NTSC) programming, the captions are “encoded” into Line 21 of the vertical blanking interval, which is a part of the broadcast picture that is positioned above the visible portion of the picture. For the Advanced Television Systems Committee (ATSC) programming, which includes digital or high-definition broadcasts, three streams are encoded in the video: two are backward compatible Line 21 captions, and the third is a set of up to 63 additional caption streams encoded in an Electronic Industries Alliance (EIA) format, the EIA-708 format. In countries that utilize a phase-alternating line or Systeme Electronique Couleur Avec Memoire (SECAM) programming formats, where teletext is used rather than Line 21, the captions are transmitted within the broadcast stream, but using a different method.
In the United States, since the passage of the Television Decoder Circuitry Act in 1990, manufacturers of most television receivers have been required to include a closed-captioning decoder to decode the captions from the broadcast signal and to provide the decoded captions for superimposed display, when closed-captioning is enabled. For older televisions that do not include a closed-captioning decoder, a set-top box or other external decoder may be required to provide closed-captioning.
NTSC DVDs may carry closed captions in the Line 21 format which are automatically sent to the TV and turned on and off by the TV remote or the set-top decoder. Video DVDs may carry closed captions as a bitmap overlay, which can be turned on and off via the DVD player.
Recently, as more consumers have subscribed to residential broadband Internet access, users are spending increasing amounts of time browsing the Internet at home, often while watching television. However, unless closed-captioning is turned on, the consumer may miss information available via the URL data within the closed-captioning information.